1. Field of the Invention
This invention relates to a method of manufacturing an SOI wafer, and thus-manufactured SOI wafer.
2. Description of the Related Art
Patent Document 1: Japanese Laid-Open Patent Publication No. H9-64319
So-called SOI wafers, obtained by forming a silicon oxide film on a silicon single crystal substrate (also referred to as “base wafer”, hereinafter), and by forming another silicon single crystal layer as being stacked thereon as an SOI (silicon on insulator) layer, are used for manufacturing semiconductor devices such as CMOS-IC and high-withstand-voltage-type IC. Bonding process, the SIMOX process and so forth are known as methods for manufacturing the SOI wafers. The bonding method is such as bonding a base wafer and a bond wafer composed of silicon single crystal while placing a silicon oxide film in between, and then thinning the bond wafer by polishing, etching, or delamination with the aid of an ion implanted layer (so-called, Smart-Cut Process (registered trademark)) so as to leave it as an SOI layer. On the other hand, the SIMOX process is such as implanting a high dose of oxygen ion into a silicon single crystal substrate, and then carrying out internal oxidation annealing to thereby form a buried oxide film.
Conventional SOI wafers have suffered from a drawback in that they are likely to cause warping of the substrate, due to difference in thermal expansion coefficients between the base wafer and the SOI layer composed of silicon single crystal, and the silicon oxide layer. If the warping of the SOI wafer increases, focusing in photolithographic process may become difficult, and device formation consequently becomes difficult. The drawback will be more distinct as the degree of integration of integrated circuits increases.
Various measures have conventionally been proposed for the above-described warping of the SOI wafers, while paying attention mainly to warping that possibly occurs in the bonding annealing in the above-described bonding process, or in the internal oxidation annealing of the SIMOX process. For example, Patent Document 1 discloses a structure of an SOI wafer, having a defect-free layer with zero density of formation of oxygen precipitate, formed in the base wafer in a region thereof in contact with the silicon oxide film, whereas the residual region of the wafer remains as an oxygen precipitate layer having a density of oxygen precipitate larger than in the defect-free layer, stating that the warping of the wafer ascribable to the above-described difference in the thermal expansion coefficients is avoidable.